A selective catalytic reduction NOx catalyst (hereinafter, simply referred to as a “NOx catalyst”) is known which uses ammonia as a reducing agent to purify NOx contained in exhaust gas from an internal combustion engine. Ammonia or an ammonia precursor (for example, urea) is supplied to exhaust gas on an upstream side of the NOx catalyst. In addition, when determining whether or not purification of NOx at the NOx catalyst is normally performed or when determining whether or not the NOx catalyst is in a normal state, an output value of a NOx sensor provided on a downstream side of the NOx catalyst is sometimes used.
It is known that, when the output value of the NOx sensor provided on the downstream side of the NOx catalyst shows a downward trend regardless of an increase in an amount of NOx flowing into the NOx catalyst, a determination is made that an ammonia slip in which ammonia flows out from the NOx catalyst has occurred and a supply amount of ammonia is excessive (for example, refer to PTL 1).
Moreover, since the NOx sensor detects ammonia as well as NOx, the output value of the NOx sensor is a value in accordance with a NOx concentration and an ammonia concentration in exhaust gas. Therefore, in a known technique, detection of NOx by a NOx sensor is stopped in consideration of the fact that detection accuracy of the NOx sensor declines when ammonia is being added (for example, refer to PTL 2).
In another known technique, two NOx catalysts are provided in series and ammonia is supplied to an upstream side NOx catalyst so that an ammonia concentration between the upstream side NOx catalyst and a downstream side NOx catalyst equals a prescribed concentration (for example, refer to PTL 3).
Moreover, as a NOx catalyst deteriorates, ammonia adsorption performance of the NOx catalyst declines. Further, as an amount of ammonia adsorbed by the NOx catalyst decreases due to a decline in its ammonia adsorption performance, NOx purification performance declines. In addition, due to the decline in ammonia adsorption performance, an amount of ammonia that flows out from the NOx catalyst increases when ammonia is supplied to the NOx catalyst. Accordingly, in a known technique, a determination is made that a NOx catalyst has deteriorated when a difference between an actually-measured value and an estimated value of an ammonia concentration on a downstream side of the NOx catalyst is equal to or larger than a prescribed value (for example, refer to PTL 4).